GPS

The GPS system over our heads is so accurate that it measures in centimetres. You might be surprised to hear that the position of the British Isles changes from day to day. You've no doubt heard of continental drift. We are drifting towards Norway at the same speed that your fingernails grow. That might seem slow to you but in geological time that's quite fast. Give it a few hundred million years and you just look out the window!
Also we are a little set of islands on a broad continental shelf. When the tide comes in twice a day that's a huge body of water propelled by the gravity of the moon. The weight of all that water presses down on the shelf and causes it to push the land-mass down into the underlying magma of the earths underbelly and the satellites can pick up on this and measure the centimetres of difference.
It's a wonder we are all not sea-sick.